A method in which some of exhaust gas is taken out of the exhaust system of a diesel engine, and is returned again to the air intake system and is added to an air-fuel mixture is called EGR (Exhaust Gas Recirculation). This method has been widely used as an effective method for purifying exhaust gas of diesel engine, and for improving the heat efficiency because many effects can be achieved, for example, the occurrence of NOx (nitrogen oxides) can be restrained, the loss of heat released to a coolant due to a decrease in pump loss and a lowering temperature of combustion gas is reduced, the ratio of specific heat is increased by a change of quantity and composition of working gas, and the cycle efficiency is accordingly improved.
If the temperature of EGR gas increases, and the quantity of EGR increases, however, the durability of EGR valve is deteriorated by the heat action of EGR gas, and the EGR valve may be broken at an early stage. Therefore, a cooling system must be provided to form a water cooling structure as preventive measures, or there occurs a phenomenon that the filling efficiency is decreased by the increase in intake air temperature and hence the fuel economy is decreased. To avoid such circumstances, a device for cooling the EGR gas using an engine cooling fluid, a refrigerant for air conditioner, or cooling air has been used. In particular, a large number of EGR gas cooling systems of a gas-liquid heat exchange type, which cool the EGR gas using the engine cooling fluid, have been proposed and used. Among these EGR gas cooling systems of a gas-liquid heat exchange type, an EGR gas cooling system of double-tube heat exchange type has still been demanded strongly. A large number of double-tube heat exchangers have been proposed including, for example, a double-tube heat exchanger in which an outer tube for allowing a liquid to pass through is disposed on the outside of an outer tube for allowing a high-temperature EGR gas to pass through, and in a heat exchanger for accomplishing heat exchange between gas and liquid, a metallic corrugated plate is inserted as a fin in the inner tube (for example, refer to Japanese Patent Laid-Open Publication No. 11-23181 (FIGS. 1 to 4)), and a double-tube heat exchanger which is formed by an inner tube for allowing a cooled medium to flow on the inside, an outer tube provided so as to surround the inner tube so as to be separated from the outer periphery of the inner tube, and a radiation fin having a thermal stress relaxing function that is provided in the inner tube (for example, refer to Japanese Patent Laid-Open Publication No. 2000-111277 (FIGS. 1 to 7)).
According to the double-tube heat exchanger incorporating a fin structure on which improvement has been made in various manners as described above, although the construction is simple and compact, a high cooling efficiency can be anticipated as such. Therefore, as a heat exchanger for cooling EGR gas that is used in a limited installation space such as a small-sized automobile, many double-tube heat exchangers have already been used practically. However, because of its compact construction, the absolute quantity of flowing fluid has a limit naturally. As a result, unsolved problems are remained in terms of the total heat exchange efficiency. In order to solve such problems, what is called a heat exchanger of a shell-and-tube type must inevitably adopted although the construction is somewhat complicated and large. The heat exchanger of this type has also been improved in various manners. As one example of the heat exchanger of a shell-and-tube type, a heat exchanger has been disclosed in which a cooling water inlet is provided at one end of the outer peripheral portion of a shell body forming a cooling jacket, and a nozzle serving as a cooling water outlet is provided at the other end thereof; a bonnet for introducing high-temperature EGR gas is integrally provided at one end in the lengthwise direction of the shell body, and a bonnet for exhausting heat-exchanged EGR gas is integrally provided at the other end thereof; a plurality of flat heating tubes are installed at intervals via a tube seat attached to the inside of the bonnet; the high-temperature EGR gas flows in the flat heating tubes so as to cross the cooling water flowing in the shell body; and a plate fin having a U-shaped cross-sectional shape is incorporated on the inner peripheral surface of the flat heating tube, by which the flow of flowing EGR gas is made a small stream, and at the same time, the heat transfer area is further increased, thereby providing a high heat exchange efficiency (for example, refer to Japanese Patent Laid-Open Publication No. 2002-107091 (FIGS. 1 to 3)).
On the other hand, in the above-described heat exchanger of a shell-and-tube type, to improve the heat exchange efficiency, it is an essential requirement to allow EGR gas, which is the cooled medium, to flow with uniform flow rate distribution and flow velocity in each heating tube that is disposed in large numbers at intervals in the shell to form a heating tube group, and at the same time, to produce a turbulent flow and agitating action appropriately between the fluids, which are the cooled medium and the cooling medium. According to the EGR gas cooling system shown in FIG. 9A, a flat heating tube 10 for heat exchanger has been proposed in which a heating tube that is disposed in large numbers in a shell body 30 forming a cooling jacket to form a heating tube group is a flat heating tube 10 consisting of a bottom portion 10-6 and an upper lid portion 10-5; as shown in FIG. 9B, a corrugated fin 20 having a substantially rectangular channel-shaped cross section and having waveform meandering 20-1 at predetermined intervals in the lengthwise direction is incorporated; and also, a turbulent flow forming portion 10-1 with respect to the gas flow is formed by providing a plurality of concave portions 10-3 and convex portions 10-2 on an exhaust gas flow path 10-4 in the flat heating tube 10 (for example, refer to Japanese Patent Laid-Open Publication No. 2004-263616 (FIGS. 1 to 10)). Also, a report has been made such that a periodic turbulent flow is produced in the EGR gas flowing in a gas flow path 10-4 in the flat heating tube 10 to effectively prevent the adhesion of soot, and the cooling medium such as cooling water flowing on the outer peripheral surface of the heating tube 10 is also agitated effectively, by which the heat exchange performance between gas and liquid is enhanced. Also, in the heat exchanger shown in FIG. 10A, a heat exchanger 40a for cooling exhaust gas in which an exhaust gas flow path 30a-1 is formed so as to have a flat cross-sectional shape and is laminated in a plurality of tiers is shown. In the flat exhaust gas flow path 30a-1, a corrugated fin structure 20a having a substantially rectangular channel-shaped cross-sectional plane as shown in FIG. 10C and having meandering in the lengthwise direction as shown in FIG. 10B is inserted. Thereby, a heat exchanger having a construction substantially similar to Japanese Patent Laid-Open Publication No. 2004-263616 (FIGS. 1 to 10) has been disclosed. The corrugated fin structure 20a in this example is formed so that, as shown in FIGS. 10B and 10D, the period of waves corresponding to the wave meandering viewed in a plan view, namely, the periods of peak lines 20a-3 and valley lines 20a-4 are longer than the period T2 on the outlet side 20a-6 of gas as compared with the period T1 on the inlet side 20a-7 of gas, and the corrugated fin structure 20a is inserted in the flat exhaust gas flow path 30a-1, by which a heat exchanger in which a gas flow path substituting the flat heating tube incorporating the corrugated fin is used has been proposed (for example, refer to Japanese Patent Laid-Open Publication No. 2004-177061 (FIGS. 1 to 4)). A report has been made such that by making the period of waves on the exhaust gas outlet side longer than that on the inlet side and a gentle curve, the flow of gas is accelerated and hence the accumulation of soot is prevented, and at the same time, the agitation of fluid is promoted and hence the heat exchange performance is enhanced.
In the above-described conventional arts, in the case of the double-tube EGR gas cooling system disclosed in Japanese Patent laid-Open Publication No. 11-23181 (FIGS. 1 to 4) and Japanese Patent Laid-Open Publication No. 2000-111277 (FIGS. 1 to 7), although the construction is simple and compact, a high cooling efficiency can be anticipated as such. Therefore, as a heat exchanger for cooling EGR gas that is used in a limited installation space such as a small-sized automobile, many double-tube heat exchangers have already been used practically. However, because of its compact construction, the absolute quantity of flowing fluid has a limit naturally. As a result, unsolved problems are remained in terms of the total heat exchange efficiency.
To solve the above-described problems, in the heat exchanger type EGR gas cooling system of a shell-and-tube type described in Japanese Patent Laid-Open Publication No. 2002-107091 (FIGS. 1 to 3) and Japanese Patent Laid-Open Publication No. 2004-177061 (FIGS. 1 to 4), improvement has been made such that the heat exchanger tube is made a flat heating tube having a larger heat transfer area, and the fin structure having a U-shaped cross section is incorporated in the flat heating tube; the corrugated fin incorporated in the flat heating tube is made a waveform having a substantially rectangular channel-shaped cross section and the corrugated fin is formed with waveform meandering in the lengthwise direction, and in addition, a plurality of irregularities are provided on the fluid flow path surface of the flat heating tube to form a turbulent flow forming portion; or the period of meandering in the lengthwise direction of the corrugated fin incorporated in the flat gas flow path in the laminated heat exchanger is made longer on the outlet side as compared with the period on the gas inlet side. Reports have been made such that by making improvement as described above, the accumulation of soot in the tube was prevented by producing a turbulent flow appropriately in the flow of EGR gas flowing in the gas flow path in the heating tube, or the agitating action of the cooling medium such as cooling water flowing on the outside of the heating tube was promoted, by which high heat exchange performance between gas and liquid was obtained, and some conventional arts have already been used practically. Actually, however, concerning the shape of wave as the corrugated fin structure that is incorporated in the flat heating tube and can effectively promote heat exchange between the high-temperature fluid flowing in the tube and the cooling medium flowing on the outside of the tube, the optimization has not yet been achieved. Therefore, substantially, a sufficient performance cannot be obtained, and room for further improvement is left.
More specifically, in the case where the heat transfer area in the heating tube is small, an attempt is made to enhance the heat transfer performance by increasing the flow velocity. In this case, however, the pressure loss increases inversely, and in addition, the adhesion of soot and dirt to the interior of flow path deteriorates the performance because an attempt is made to enhance the heat transfer performance by increasing the flow velocity. In the case where the number of heating tubes is increased to reduce the pressure loss, the heat transfer performance per one heating tube decreases, so that the volume of the heat exchanger itself increases to secure the initial performance. Therefore, there arise new problems of, for example, a serious hindrance in terms of layout.